Patent Document 1 discloses a conventional flush tank device. The flush tank device includes a tank body, a water supply device and an air suction device which sucks air in a toilet drain duct. The tank body has a drain outlet which is capable of being opened and closed by an on-off valve and of storing flush water used to flush a toilet body. The drain outlet of the tank body communicates with the toilet body having a toilet bowl and the toilet drain duct communicating with a downstream side of the toilet bowl. The water supply device supplies flush water into the tank body. The air suction device is disposed in the tank body and comprises a booth having a closed upper end and an open lower end and an air suction duct provided in the booth and extending over an uppermost water level in the tank body. The air suction duct communicates with the toilet drain duct.
In this flush tank device, when the on-off valve of the drain outlet is opened so that toilet flushing is started, the flush water stored in the tank body flows through the drain outlet into the toilet body. As a result, a water level of the flush water stored in the tank body lowers. In this case, air in the toilet drain duct is sucked through the air suction duct into the booth of the air suction device such that the flush water level lowers. More specifically, the air suction device sucks air in the toilet drain duct according to lowering of the flush water level in the booth. The air suction device sucks air in the toilet drain duct at the time of toilet flushing, whereby a siphon action is reliably caused in the toilet drain duct, with the result that the siphon action can be facilitated.